It is known that Bragg gratings impressed in optical fibers may be used to detect perturbations, such as strain or temperature, at the location of the gratings, as is described in U.S. Pat. Nos. 4,806,012 and 4,761,073, both to Meltz et al. In such a sensor, the core of the optical fiber is written with periodic grating patterns (i.e., a periodic variation in refractive index) effective for reflecting a narrow wavelength band of light launched into the core. Spectral shifts in the transmitted and reflected light indicate the intensity of strain or temperature variations at positions of the grating corresponding to the associated wavelengths of the injected light. When such a sensor is embedded in or disposed on a structure for measuring strains and loads on the structure, the structure is called a "smart structure system".
However, such Bragg grating sensors require a spectrometer to determine the sensor response, to determine the wavelength shift for each of the gratings, and to multiplex from one grating to the next. Furthermore, such a spectrometer-based system is costly, delicate, bulky, and has a slow response time.
One technique known in the art for eliminating the need for a spectrometer is described in a paper by S. Melle, "Strain Sensing Using A Fibre Optic Bragg Grating", Institute for Aerospace Studies, University of Toronto, Downsview, Ontario, M3H 5T6, Canada. Such a technique uses a wavelength dependent optical filter having a frequency response which has a linear region above which all wavelengths are passed (or transmitted) through the filter. The reflective peak of the Bragg grating is designed to be located within the linear attenuation region of the filter. Thus, as the central wavelength of the reflectivity profile (or spectrum) of the Bragg grating shifts according to strain, temperature or other perturbation, the output intensity of the filter will change accordingly. However, because the filter passes all wavelengths above the linear region, only a single Bragg grating sensor may be used with each optical filter.
Thus, it would be desirable to provide a system which completely replaces a spectrometer and allows a plurality of Bragg grating sensors on a single fiber as well as a plurality of fibers, each having a plurality of Bragg gratings to be detected, and which senses each of the wavelengths (and shifts thereof) reflected by the Bragg grating.